In long-standing practice, as shown for example in commonly-assigned U.S. Pat. No. 3,733,384, plastic containers are formed by introducing a molten plastic tube (parison) between spaced dies and closing the dies upon the parison. The dies define a cavity which imparts desired exterior shape to the parison, inclusive of a cavity part providing a container filling/emptying opening. A blow pin is supported in the paths of movement of the parison to be sealably encircled by the parison on closure of the dies. As the dies close, the blow pin conducts pressurized air into the parison, forcing it to conform to the shape of the die cavity.
A measure of dimensional stability of the thus-formed container need of course to be attained prior to withdrawing the container from the dies, i.e., the plastic substance need be cooled to effect a transition from its flowable parison state to a substantially less flowable condition. For this purpose, coolant is continually circulated through the dies to extract heat from the plastic material.
The time of closure of the dies is the dominant factor in the commercially-significant cycle time for making a plastic blow-molded container. Thus, the longer the die closure time prior to reaching sufficient container dimensional stability to permit die opening, the lesser the number of containers produced per unit time. With respect to dimensional stability, post-forming shrinkage occurs in increasing measure with increased temperature of the plastic material on die-opening. Lessening of die closure time and of container temperature on die-opening have accordingly been primary objectives of commercial container producers.
In seeking after these objectives, artisans have looked toward cooling interiorly of the formed parison as a supplement to the above-noted die cooling practice. One prior art effort attaining the objectives, however, at substantial cost increase, involves the introduction of liquid carbon dioxide through the blow pin. On its presence in the high temperature parison, the carbon dioxide undergoes state change to a solid, and then sublimes, thereby affecting substantial cooling of the parison.
Another prior art effort provides for the use of water as a direct coolant for the parison. The water is sprayed onto the parison interior surface through a spray head supported within the blow pin. This technique, known as the ISC system, applies water droplets onto the parison interior surface and such droplets have been observed to be non-uniformly distributed and to mar the interior walls of the formed container. There also is an undesired water remnant in the container.
U.S. Pat. No. 3,694,424 describes an improved ISC practice said to be effective to uniformly cost the parison interior. A chilled highly-pressurized mixture of air and water is injected into the parison in the course of its customary air pressurization. The mixture is pressurized at a level some ten times more than such customary level. The patent hypothesizes that the chilled mixture is subjected to such sudden pressure drop that there is an instantaneous adiabatic expansion of the mixture applying water in its three phases to all interior surface of the parison. Increased costs in this practice attend the chilling and high pressurization of the air-water mixture.